Add config file so that we can get continuous integration from Travis, and put a build indicator in the README.

Results will show up at https://travis-ci.org/google/closure-compiler
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=70666361
Fix null scope in Es6HandleDefaultParameters
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=70666728